About Cache Type and Size

EclipseLink provides several different cache types which have different memory requirements. The size of the cache (in number of cached objects) can also be configured. The cache type and size to use depends on the application, the possibility of stale data, the amount of memory available in the JVM and on the machine, the garbage collection cost, and the amount of data in the database.

By default, EclipseLink uses a SOFT_CACHE with an initial size of 100 objects. The cache size is not fixed, but just the initial size, EclipseLink will never eject an object from the cache until it has been garbage collected from memory. It will eject the object if the CACHE type is used, but this is not recommended. The cache size of the SOFT_CACHE and HARD_CACHE is also the size of the soft or hard sub-cache that can determine a minimum number of objects to hold in memory.

You can configure how object identity is managed on a class-by-class basis. The ClassDescriptor object provides the cache and identity map options described in Table 9-1.

There are two other options, NONE, and CACHE. These options are not recommend.

FULL Cache Type

This option provides full caching and guaranteed identity: objects are never flushed from memory unless they are deleted.

It caches all objects and does not remove them. Cache size doubles whenever the maximum size is reached. This method may be memory-intensive when many objects are read. Do not use this option on batch operations.

Oracle recommends using this identity map when the data set size is small and memory is in large supply.

WEAK Cache Type

This option only caches objects that have not been garbage collected. Any object still referenced by the application will still be cached.

The weak cache type uses less memory than full identity map but also does not provide a durable caching strategy across client/server transactions. Objects are available for garbage collection when the application no longer references them on the server side (that is, from within the server JVM).

SOFT Cache Type

This option is similar to the weak cache type, except that the cache uses soft references instead of weak references. Any object still referenced by the application will still be cached, and objects will only be removed from the cache when memory is low.

The soft identity map allows for optimal caching of the objects, while still allowing the JVM to garbage collect the objects if memory is low.

SOFT_CACHE and HARD_CACHE Cache Type

These options are similar to the weak cache except that they maintain a most frequently used sub-cache. The sub-cache uses soft or hard references to ensure that these objects are not garbage collected, or only garbage collected only if the JVM is low on memory.

The soft cache and hard cache provide more efficient memory use. They release objects as they are garbage-collected, except for a fixed number of most recently used objects. Note that weakly cached objects might be flushed if the transaction spans multiple client/server invocations. The size of the sub-cache is proportional to the size of the cache as specified by the size option. You should set the cache size to the number of objects you wish to hold in your transaction.

Oracle recommends using this cache in most circumstances as a means to control memory used by the cache.

NONE and CACHE

NONE and CACHE options do not preserve object identity and should only be used in very specific circumstances. NONE does not cache any objects. CACHE only caches a fixed number of objects in an LRU fashion. These cache types should only be used if there are no relationships to the objects.Oracle does not recommend using these options. To disable caching set the cache isolation to ISOLATED instead.

About the Internals of Weak, Soft, and Hard Cache Types

The WEAK and SOFT cache types use JVM weak and soft references to ensure that any object referenced by the application is held in the cache. Once the application releases its reference to the object, the JVM is free to garbage collection the objects. When a weak or a soft reference is garbage collected is determined by the JVM. In general, expect a weak reference to be garbage collected with each JVM garbage-collection operation.

The SOFT_CACHE and HARD_CACHE cache types contain the following two caches:

Reference cache: implemented as a LinkedList that contains soft or hard references, respectively.

Weak cache: implemented as a Map that contains weak references.

When you create a SOFT_CACHE or HARD_CACHE cache with a specified size, the reference cache LinkedList is exactly this size. The weak cache Map has the size as its initial size: the weak cache will grow when more objects than the specified size are read in. Because EclipseLink does not control garbage collection, the JVM can reap the weakly held objects whenever it sees fit.

Because the reference cache is implemented as a LinkedList, new objects are added to the end of the list. Because of this, it is by nature a least recently used (LRU) cache: fixed size, object at the top of the list is deleted, provided the maximum size has been reached.

The SOFT_CACHE and HARD_CACHE are essentially the same type of cache. The HARD_CACHE was constructed to work around an issue with some JVMs.

If your application reaches a low system memory condition frequently enough, or if your platform's JVM treats weak and soft references the same, the objects in the reference cache may be garbage-collected so often that you will not benefit from the performance improvement provided by it. If this is the case, Oracle recommends that you use the HARD_CACHE. It is identical to the SOFT_CACHE except that it uses hard references in the reference cache. This guarantees that your application will benefit from the performance improvement provided by it.

When an object in a HARD_CACHE or SOFT_CACHE is pushed out of the reference cache, it gets put in the weak cache. Although it is still cached, EclipseLink cannot guarantee that it will be there for any length of time because the JVM can decide to garbage-collect weak references at anytime.